Influence of trapped and interfacial charges in organic multilayer light-emitting devices

Trapped and interfacial charges have significant impact on the performance of organic light-emitting devices (OLEDs). We have studied devices consisti ng of 20 nm copper phthalocyanine (CuPc) as the buffer and hole-injection l ayer, 50 nm N,N'-di(naphthalene-l -yl)-N,N'-diphenyl-benzidine (NPB) as the hole transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq(3) ,) as the electron transport and emitting layer sandwiched between a high-w ork-function metal and a semitransparent Ca electrode. Current-voltage meas urements show that the device characteristics in the negative bias directio n and at low positive bias below the built-in voltage are influenced by tra pped charges within the organic layers. This is manifested by a strong depe ndence of the current in this range on the direction and speed of the volta ge sweep. Low-frequency capacitance-voltage and static charge measurements reveal a voltage-independent capacitance in the negative bias direction and a significant increase between 0 and 2 V in the given device configuration , indicating the presence of negative interfacial charges at the NPB/Alq(3) interface. Transient experiments show that the delay time of electrolumine scence at low voltages in these multilayer devices is controlled by the bui ldup of internal space charges, which facilitates electron injection, rathe r than by charge-carrier transport through the organic layers. To summarize , our results clearly demonstrate that the tailoring of internal barriers i n multilayer devices leads to a significant improvement in device performan ce.